Decode function

Hi,
Can i use two column names in a single decode function...

for eg
select sum(decode(A.TableA,20,B.TableB,0)) .....

I was analyzing a query just similar to above select stmt. Was just wondering why they have used two columns in one decode function...

can somebody put some light on this...

It does sum(value) where value is defined as:
if A.TableA = 20 then value=B.TableB
else value=0